dd(1) is a utility for writing and reading files or devices. It's available in macOS by default. You can make bootable disk images using this without external tools.

1) Attach your USB drive, and in a Terminal, find your device:

$ diskutil list
...
/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     Apple_partition_scheme                        *7.7 GB     disk4
   1:        Apple_partition_map                         4.1 KB     disk4s1
   2:                  Apple_HFS                         5.2 MB     disk4s2
                    (free space)                         7.7 GB     -

Look for (external, physical). The string preceding it is your device name. In this case, it's /dev/disk4. We'll refer to this as $DISK_ID – in any of the steps below, replace $DISK_ID with your device's name from here.

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A simple way to verify that you have the correct device name is to unplug your drive -> run diskutil list -> plug it in -> run diskutil list again, and compare the difference. You should see your new device in the second output of diskutil list that isn't in the first output.

2) Using the device name from above, wipe the drive:

❗
Double-check the device name to avoid wiping the wrong drive!
$ sudo dd if=/dev/zero of=$DISK_ID bs=1M count=10

This will wipe the first 10MB of the drive which removes any partition/filesystem data rendering it unmountable and unreadable. It does not wipe the whole drive because the subsequent copy of the ISO data will overwrite everything anyway.

3) Format the drive:

# Replace $DRIVE_NAME with a name of your choice. You can use any name.

# Use this for Windows ISOs to format it as FAT32 using Master
# Boot Record scheme:
diskutil eraseDisk FAT32 $DRIVE_NAME MBR $DISK_ID

# Use this for all other ISOs (Linux, etc.) to format it as
# FAT32 using GUID Partition Table scheme:
diskutil eraseDisk FAT32 $DRIVE_NAME GPT $DISK_ID

4) Un-mount the drive:

$ diskutil unmountDisk $DISK_ID

5) Write your ISO to the drive:

$ sudo dd if=/path/to/iso.iso of=$DISK_ID bs=1m

You're done. No external applications or third-party tools required.